One method of assessing the useful life of pressure tubes in nuclear reactors, such as a CANDU reactor, requires the periodic removal of a tube. Samples are cut from the removed tube and analyzed for deuterium content. The deuterium concentration is then used as a measure of the useful life of the remaining pressure tubes. This approach is very costly because of the long shutdown period required to remove and replace a pressure tube.
Attempting to provide in-situ sampling (without pressure tube removal) presents numerous difficulties. Obtaining a useful sample is made difficult by the hard oxidized surface, and the need to obtain sample material from beneath the surface layer. To preserve the structural integrity of the tube and avoid detrimental residual stress, the sampling depth must be controlled and the sampled region must be left with smooth changes in geometry in all axes. Furthermore, the technique used for removing the surface material or sample must not involve excessive heating, as this affects the results of the subsequent analysis. Another difficulty is the recovery of the sample for analysis and preventing particles from being left in the pressure tube.
U.S. Pat. No. 4,925,621, issued May 15, 1990, the entirety of which is incorporated herein by reference, discloses a sampling tool useful for pressure tube sampling which addresses the above difficulties. The disclosed sampling tool permits in situ testing in that pressure tube removal is unnecessary. The sampling tool comprises two cutters and means for capturing the removed material. By moving both cutters axially in the pressure tube, one cutter removes the surface oxide layer, and the second cutter removes a sample for analysis. The cutters and cutting operation are designed to avoid damaging the integrity of the pressure tube to allow it to remain in service.
Although the above-described sampling tool addresses the above difficulties, it proves impractical to obtain samples in some portions of the pressure tube. For example, as seen in FIG. 1, in a CANDU type fuel channel, the pressure tube 10 is joined to an end fitting (not shown) using a rolled joint 12. The above-described sampling tool makes obtaining useful samples in the rolled joint area difficult due to the high axial gradient of hydrogen/deuterium concentration and the circumferential ripples 14 in the rolled joint area.
The conference paper presented at the 5th International CANDU Maintenance Conference in November 2000 which is entitled “Advanced Pressure Tube Sampling Tools” and is authored by K. Wittich and J. King also discloses sampling tools. The conference paper presented at the 7th International CANDU Maintenance Conference in November 2005 which is entitled “Innovation in Pressure Tube Rolled Joint Sampling (Circumferential Sampling Tool Technology)” and is authored by B. Guler, J. King, and R. Wray also discloses sampling tools. Both papers are published by the Canadian Nuclear Society.
Therefore, there is a need for a sampling tool that addresses at least some of the above-identified difficulties and at least some of the inconveniences present in the prior art.